Process for manufacture of improved needle coke from petroleum

ABSTRACT

AN IMPROVED COKE FOR THE PRODUCTION OF GRAPHITE ELECTRODES IS PRODUCED FROM A PETROLEUM COKING FEEDSTOCK CONTAINING 0.01% BY WEIGHT OR MORE OF SPENT CRACKING CATALYST BY REDUCING THE SPENT CRACKING CATALYST CONTENT OF THE PETROLEUM COKING FEEDSTOCK TO LESS THAN 0.005% BY WEIGHT PRIOR TO THE COKING OF SAID FEEDSTOCK. WHEN A SMALL AMOUNT OF COLLOIDAL GRAPHITE IS ADDED TO THE PETROLEUM COKING STOCK HAVING LESS THAN 0.005% BY WEIGHT OF SPENT CRACKING CATALYST JUST BEFORE IT IS COKED, AN ADDITIONAL IMPROVEMENT IN THE COKE IS OBTAINED.

United States Patent 3,704,224 PROCESS FOR MANUFACTURE OF IMPROVEDNEEDLE COKE FROM PETROLEUM Warner E. Scovill, Lakewood, and Donald R.Day, Garfield Heights, Ohio, assignors to The Standard Oil Company,Cleveland, Ohio No Drawing. Filed Oct. 2, 1970, Ser. No. 77,735

Int. Cl. Cg 9/14 US. Cl. 208-131 3 Claims ABSTRACT OF THE DISCLOSURE Animproved coke for the production of graphite electrodes is produced froma petroleum coking feedstock containing 0.01% by Weight or more of spentcracking catalyst by reducing the spent cracking catalyst content of thepetroleum coking feedstock to less than 0.005% by weight prior to thecoking of said feedstock. When a small amount of colloidal graphite isadded to the petroleum coking stock having less than 0.005% by weight ofspent cracking catalyst just before it is coked, an additionalimprovement in the coke is obtained.

The present invention relates to a process for the manufacture ofimproved needle coke and more particularly pertains to a process forpreparing a petroleum coking feedstock which produces a superior cokefor the production of graphite electrodes and the like.

Coke is produced from petroleum by well known methods and certain ofthis coke has been referred to as needle coke (see US. Pat. No.2,775,549, for instance). Needle coke is particularly suited for use inthe manufacture of graphite electrodes. Needle coke is producedgenerally by a procedure known as delayed coking. The quality of needlecoke for the manufacture of metallurgical electrodes is conventionallydetermined by measuring the properties of the finished graphiteelectrode. Properties such as flexural strength, coefficient of thermalexpansion, and electrical resistivity are of importance, although theproperty most critical and usually measured to determine the quality ofthe needle coke is the coefficient of thermal expansion (CTE).

It is generally accepted by those skilled in the art that preferredfeedstocks for the manufacture of needle coke are highly aromatic incharacter and include such stocks as slurry and decanted oils fromcatalytic cracking and tars from thermal cracking. The stocks derivedfrom catalytic cracking inherently contain varying amounts of spentcatalyst fines. The spent catalyst fines, while mainly inorganic, havean amorphous carbon surface. Thermal tars can also contain spentcatalyst fines when such stocks as slurry and decanted oils are used asfeed to the thermal cracker. We have discovered that the presence offine particulate spent catalyst in coker feed exerts a deleteriouseffect on needle coke structure which is reflected in poor CTE ingraphite electrodes produced therefrom.

It is an object of this invention to provide an improved quality cokefrom a coker feedstock from the catalytic cracking process by removingmost of the fine spent catalyst particles suspended in the cokerfeedstock by suitable means such as centrifuging, filtering, and thelike. It is another object of this invention to recycle the spentcatalyst fines removed from the coker feedstock to the cracking catalystregenerator and back into the cracking system. It is another object ofthis invention to further improve the quality of needle coke by seedingthe coker feedstock, which is essentially free of fine particulatematter, with a material of proper structure such as finely dividedcolloidal graphite particles. It is believed that the inclusion of alarge number of finely divided graphite particles (usually less than onemicron in size) into the coker feedstock just prior to coking promotesthe formation of needle coke having a more ordered structure whichultimately will produce a graphite electrode having improved CTE andother desirable properties.

A typical catalytic cracking process is more completely disclosed in US.Pat. No. 3,129,107 and in Petroleum Refiner, September 1966, page 187.The feedstock preferred for the production of needle coke and ultimatelygraphite electrodes is called clarified slurry in the Petroleum Refinerarticle referred to above, and this material is also known to thoseskilled in the art as decanted oil or clarified oil. Decanted oil isproduced from the top of the slurry settler and in most instancescontains in the range of 0.01 to 1% by weight of spent crackingcatalysts (such as silica-alumina) fines (typical 2-50 micron sizerange) suspended therein. We have found that decanted oil containingspent catalyst fines in this range produces a coke and ultimately agraphite electrode which is inferior to the coke and graphite electrodesproduced by a decanted oil containing less than 0.01% by weight of spentcatalyst fines. In accordance with our process, we remove spent catalystfines from such a decanted oil by centrifugation or filtration of theoil so as to reduce the spent catalyst fines level in the decanted oilto be used as coker feedstock to a level about 0.005 by weight. Inaccordance with our process, the coke and graphite electrodes producedtherefrom are of superior quality.

Our process includes the delayed coking of the decanted oil containingless than 0.005 by weight of spent catalyst fines. Delayed coking ismore fully describedd in Petroleum Refiner, September, 1966, page 191.In our process, the decanted oil containing less than 0.005 by weight ofspent catalyst fines is heated and charged to the lower portion of afractionator. Here the charge meets the hot vapors from the coking drumand light components are flashed off. The heavy residue passes from thebottom of the fractionator to a furnace Where it acquires the heat ofcracking. Then the heated residue is introduced into an insulated drumWhere the residence time is suflicient for coke to form and settle fromthe mixture. The vapors from the coking drum return to the fractionator.Here the gas, gasoline, and gas oil are separated and leave the unit.The heavier materials appear in the bottom of the fractionator and arerecycled to the coking operation. When coke builds up to a predeterminedlevel in one of the coking drums, fiow is diverted to another drum sothat the furnace operation is continuous. Thus, drums are operated inpairs with one on-stream while the other is being decoked. A full cokedrum is removed from the process flow, steamed to strip lighthydrocarbons from the coke, and cooled by water injection. More recentdesigns use high pressure (over 1000 p.s.i.g.) water jets to cut thecoke from the drum. When graphite seeding is used according to ourinvention, the fine colloidal graphite particles are added to the oil asit passes from the furnace to the coke drum in the delayed cokingprocess. It is preferred to add from about 0.2 to 20.0 parts per millionby weight of graphite to the oil at this stage. In accordance with ourprocess, the needle coke from the delayed coking process is calcined atabout 1300 C. using known procedures to devolatilize, dehydrogenate, anddensify the coke. For more details concerning the calcination ofpetroleum coke, see Petroleum Products Handbook, Sec tion 14 onPetroleum Coke, by S. W. Martin, page 14-1.

The calcined needle coke produced by our process usually is used in theproduction of graphite electrodes by procedures well known to thoseskilled in the art. One description of such a process appears inIndustrial and Engineering Chemistry, January, 1954, pages 2-11, andparticularly page 9. In this process, calcined petroleum needle coke andcoal tar pitch are mixed together and the mixture is extruded in theshape of the desired electrode. The extruded shape is baked in an oven(about 950 C.) and passed into a graphitizer, which is an electricfurnace 'which operates at about 2800 C. Graphitizing is a treatmentwhich converts the relatively hard coke into graphite. The electrodesare removed from the graphitizing furnace and are machined to theirfinal dimensions.

The coefiicient of thermal expansion (CTE) for the finished graphiteelectrodes is determined by a procedure described in a publication ofthe US. Department of Commerce entitled Research and Development onAdvanced Graphite Materials, volume XXXVI, August 1964, produced by theAir Force Materials Laboratory, Research and Technology Division, AirForce Systems Command, Wright-Patterson Air Force Base, Ohio, at page20. The CT E is determined by measuring the linear expansion of thegraphite electrode when the temperature is raised from 30 to 100 C. Themeasurement is made parallel to the direction of preferred orientation(with the grain of the extruded electrode) and parallel to the moldingdirection. The difference in expansion between the electrode and anInvar standard is measured by means of an optical lever. The CTE isexpressed in inches per inch per degree centigrade X The lower valuesfor CTE are most desirable and represent the best graphite electrodes.In high-temperature operations such as the electrothermic furnace,increases in temperature cause significant dimensional changes ofstructural shapes and create serious stresses. The stresses can be ofsuch magnitude as to cause spalling or even gross failure of thestructural shape. Thus, in hightemperature applications, a graphiteelectrode that exhibits little or no change in dimensions withtemperature variations (low CTE) has greater durability. Typical CTEvalues for graphite produced from regular-grade coke are 12-20inches/inch/ C. 10- (US. Pat. No. 3,451,921). Typical CTE values forgraphite produced from conventional needle coke are 6.0 inches/inch/ C.10-' (US. Pat. No. 3,451,921) and 0.51 inches/inch/ C. 10 (US. Pat. No.2,922,755).

The spent catalyst content of a given decanted oil sample is determinedby a gravimetric Millipore procedure in which a one-pint sample of thedecanted oil is weighed, diluted with two volumes of toluene, andfiltered through a 1.2 micron Millipore filter. The Millipore filter isweighed, the filter apparatus is set up, and the toluene solution isfiltered. The filter is dried and weighed and this weight is noted asinsoluble residue for the total sample. The filter is then placed in acrucible and is ashed with a flame and placed in an oven at 750 C. foran hour. The crucible is cooled and weighed. The insoluble residue andash (spent catalyst content) for the total sample are reported.

The process of our invention will be now fully described in thefollowing illustrative examples in which the amounts of ingredients areexpressed as part by weight unless otherwise indicated.

EXAMPLE 1 (A) A conventional needle coke was prepared by a process whichis outside the scope of the present invention but will serve as acontrol experiment to illustrate the prior art. A decanted oil from acatalytic cracking operation was used which contained 0.0l50% by Weightof spent cracking catalyst solids. This decanted oil sample was coked,the coke was calcined and converted to a graphite electrode, and theelectrode was found to have a CTE of 3.1 inches/inch/ C. 10-

(B) The procedure of A was repeated except that before coking thedecanted oil was centrifuged to remove the spent cracking catalystsolids, and the decanted oil after centrifugation was found to contain0.0023% by weight of spent catalyst solids. The spent cracking catalystsolids were recycled to the regeneration zone of the catalytic crackerprocess to be regenerated and reused in the cracking process. Thisdecanted oil was coked and the coke was calcined and converted to agraphite electrode which was found to have a CTE of 2.7 inches/inch" C.10- an improvement of 13% over the conventional needle coke.

EXAMPLE 2 (A) The procedure of Example 1(A) was repeated with theexception that in the delayed coking process 2.0 parts per million offine particulate colloidal graphite having an average particle size ofless than one micron were added to the oil just before it entered thecoking drum. The resulting coke was calcined and converted to graphiteelectrodes which were found to have a CTE of 3.4 inches/inch/ C. 10-

(B) The procedure of Example 1(B) was repeated except that thecentrifuged decanted oil contained 0.0015% by weight of spent catalystand in the delayed coking process 2.0 parts per million of fineparticulate colloidal graphite were added to the oil just before itentered the coking drum. The resulting coke was calcined and convertedto graphite electrodes which were found to have a CTE of 1.7inches/inch/ C. 10- an im provement of 45% over the conventional needlecoke.

(C) The procedure of Example 1(B) was repeated except that thecentrifuged decanted oil contained 0.003% by weight of spent catalystand in the delayed coking process 0.2 part per million of fineparticulate colloidal graphite were added to the oil just before itentered the coking drum. The resulting coke was calcined and convertedto graphite electrodes which were found to have a CTE of 2.7inches/inch/ C. 10", an improvement of 13% over the conventional needlecoke.

(D) The procedure of Example 1(B) was repeated except that thecentrifuged decanted oil contained 0.0028% by weight of spent catalystand in the delayed coking process 20.0 parts per million of fineparticulate colloidal graphite were added to the oil just before itentered the coking drum. The resulting coke was calcined and convertedto graphite electrodes which were found to have a CTE of 1.9inches/inch/ C. 10-", an improvement of 39% over the conventional needlecoke.

We claim:

1. A process for preparing a superior coke for graphite electrodes froma coker feedstock of a highly aromatic character being selected from thegroup consisting of slurry and decanted oils from catalytic cracking andtars from thermal cracking containing from 0.01 to 1% by weight of solidspent catalyst comprising removing by centrifugation or filtration ofthe feedstock enough of said solid spent catalyst to produce a cokerfeed product containing no more than 0.005% by weight of solid spentcatalyst, seeding the coker feed product with from about 0.2 to 20.0parts per million by weight of fine particulate graphite and then cokingthe coker feed product by delayed coking.

2. The process of claim 1 wherein the catalyst is a cracking catalyst.

3. A process for preparing a superior coke for the production ofgraphite electrodes comprising adding from about 0.2 to 20.0 parts permillion by Weight of colloidal graphite to a coker feedstock of a highlyaromatic character being selected from the group consisting of slurryand decanted oils from catalytic cracking and tars from thermal crackingcontaining less than 0.005% by Weight of suspended solid material justprior to the delayed coking operation.

References Cited UNITED STATES PATENTS 2,775,549 12/1956 Shea 20850 53,326,796 6/1967 Muller 208-46 3,379,638 4/1968 Bloomer 208-1313,365,384 1/1968 Pasternack 208106 T OBIAS E. LEVOW, Primary Examiner 10A. P. DEMERS, Assistant Examiner U.S. C1. X.R.

